Heater designs for effecting the pyrolysis of naptha to form ethylene have operated on low heat rates where the conversion of naptha to ethylene ranges from about 12 to 26 percent. Such heaters have included two parallel coils and a two zone radiant heating section to permit varying of the heating profile. Horizontal tube type heaters have metallurgical limitations of the tube supports when firing such heaters at intense service conditions as well as presenting serious expansion problems.
In U.S. Pat. No. 3,274,978 to Palchik et al and assigned to the same assignee as the present invention, there is disclosed a short residence heater of the vertical tube type having parallel radiant heating zones with a single convention zone disposed thereabove in fluid communication with the radiant heating zones. By locating high intensity radiant burners on either side of a single row of absorbing surfaces or coils permit the attainment of high heat absorption rates at concomitant low residence times. The coil is designed for residence times of about 0.3 second, generally from about 0.2 to 0.5 second at average heat rates of 20,000 to 30,000 B.t.u./hr./sq.ft. At such low residence times, high outlet temperatures in the order of about 1500.degree. to 1550.degree. F. are required for the decomposition of the feed to form the desired olefins before coke condensation reactions become significant.
The lower limit of residence times in such tubular type heaters is limited by the maximum heat rate allowable and the practical limit of the smallness of the tube diameter. Thus, high pyrolysis temperatures are required for heavy hydrocarbon feeds with vaporization of the feed in the tube resulting in coke build-up, it being understood that high ethylene yields are achieved by high pyrolysis temperatures and short residence times.
Higher pyrolysis temperatures have been achieved utilizing transfer line exchangers wherein pyrolyzing temperatures are achieved by admixing heated solids with the hydrocarbon feed to be pyrolyzed, such as disclosed in U.S. Pat. Nos. 4,057,490 and 4,172,857 to Wynne and Pavilon, respectively. In U.S. Pat. No. 4,057,490 to Wynne, crushed oil shale is heated to a temperature of from 1300.degree. to 2500.degree. F. prior to introduction into the riser, whereas in U.S. Pat. No. 4,172,857 to Pavilon, agglomerated ash particles formed by burning particles of coal or other solid carbonaceous material are introduced into the riser reactor.
In ethylene production, it is most important to stop the reaction within a predetermined time, or to at least substantially rapidly reduce the reaction rate, in order to avoid the formation of by-products and residues resulting from secondary reactions and to thereby maximize the yield of the desired product. Thus, the hydrocarbon feed is introduced into a reaction zone at a very high throughput rate and rapidly brought to reaction temperature, and maintained at this temperature for a time period which may be on the order of a fraction of a second. Under these conditions ethylene is the primary resulting product.
If the reaction products are not cooled immediately, secondary reactions, such as polymerization, takes place with a resulting production of tars and coke and a reduction in ethylene yields. Aside from such evident disadvantages, there is the fact that if such secondary reactions are allowed to take place, the tars and coke which are produced tend to clog and block the pipelines, valves and other components of the apparatus with concomitant complicated maintenance problems and frequent plant shutdown.